Problem design a reverse polarity protection circuit

Question

I´m trying to develop a circuit that is able to control a starter motor with mosfets and also send a signal to control an external relay. I was able to control the starter motor through the help of a array of n channel mosfets connected in paralel. But since there is a huge amount of power necessary to control this motor I can't use the tradicional configuration of conecting a P channel mosfet for reverse polarity protection as shown in the imagem below, because it would be extremely expensive.

What I managed to do is ilustrated in the circuit below, I added a P channel mosfet after the flyback diode so the current of the starter motor will not flow trough it and I was able to protect the mcu and other components that are not shown in this simplified diagram.

But I would like to add this protection to the whole circuit and I'm not quite sure on how to do it, keeping a small footprint and been as cheap as possible. Any ideias?

Answer 1

First, you show PNP transistors being switched by +5 V from an MCU, not only will this not work, the base-emitter junction of the top transistor would put +14 V onto the MCU output through the 1K resistor which may not be good for the MCU.

Second, you are showing one relay already, and another answer is suggesting a second one. At that point why not use a solenoid instead of the MOSFETs, which is how they've been switching starter motors for about as long as they've been using starter motors?

If you use a solenoid, the motor will be disconnected until the solenoid is activated, so if you protect the rest of the circuit from reverse polarity the motor will also be protected as it will never be connected unless the rest of the circuit is working.

Answer 2

How about a relay? They are cheap, handle high current, and can be turned on in only one direction when a series diode is added. See below.

^{simulate this circuit – Schematic created using CircuitLab}